The Spur and the Gap in GD-1: Dynamical evidence for a dark substructure in the Milky Way halo. (arXiv:1811.03631v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Bonaca_A/0/1/0/all/0/1">Ana Bonaca</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hogg_D/0/1/0/all/0/1">David W. Hogg</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Price_Whelan_A/0/1/0/all/0/1">Adrian M. Price-Whelan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Conroy_C/0/1/0/all/0/1">Charlie Conroy</a>
We present a model for the interaction of the GD-1 stellar stream with a
massive perturber that naturally explains many of the observed stream features,
including a gap and an off-stream spur of stars. The model involves an impulse
by a fast encounter, after which the stream grows a loop of stars at different
orbital energies. At specific viewing angles, this loop appears offset from the
stream track. The configuration-space observations are sensitive to the mass,
age, impact parameter, and total velocity of the encounter, and future velocity
observations will constrain the full velocity vector of the perturber. A
quantitative comparison of the spur and gap features prefers models where the
perturber is in the mass range of $10^6,rm M_odot$ to $10^8,rm M_odot$.
Orbit integrations back in time show that the stream encounter could not have
been caused by any known globular cluster or dwarf galaxy, and mass, size and
impact-parameter arguments show that it could not have been caused by a
molecular cloud in the Milky Way disk. The most plausible explanation for the
gap-and-spur structure is an encounter with a dark matter substructure, like
those predicted to populate galactic halos in $Lambda$CDM cosmology. However,
the expected densities of $Lambda$CDM subhalos in this mass range and in this
part of the Milky Way are $2-3,sigma$ lower than the inferred high density of
the GD-1 perturber. This observation opens up the possibility that detailed
observations of streams could measure the mass spectrum of dark-matter
substructures and even identify individual substructures and their orbits in
the Galactic halo.
We present a model for the interaction of the GD-1 stellar stream with a
massive perturber that naturally explains many of the observed stream features,
including a gap and an off-stream spur of stars. The model involves an impulse
by a fast encounter, after which the stream grows a loop of stars at different
orbital energies. At specific viewing angles, this loop appears offset from the
stream track. The configuration-space observations are sensitive to the mass,
age, impact parameter, and total velocity of the encounter, and future velocity
observations will constrain the full velocity vector of the perturber. A
quantitative comparison of the spur and gap features prefers models where the
perturber is in the mass range of $10^6,rm M_odot$ to $10^8,rm M_odot$.
Orbit integrations back in time show that the stream encounter could not have
been caused by any known globular cluster or dwarf galaxy, and mass, size and
impact-parameter arguments show that it could not have been caused by a
molecular cloud in the Milky Way disk. The most plausible explanation for the
gap-and-spur structure is an encounter with a dark matter substructure, like
those predicted to populate galactic halos in $Lambda$CDM cosmology. However,
the expected densities of $Lambda$CDM subhalos in this mass range and in this
part of the Milky Way are $2-3,sigma$ lower than the inferred high density of
the GD-1 perturber. This observation opens up the possibility that detailed
observations of streams could measure the mass spectrum of dark-matter
substructures and even identify individual substructures and their orbits in
the Galactic halo.
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